1. Field of the Invention
This invention relates to the field of semiconductor apparatus, and more particularly to optically controlled semiconductor devices for high speed switching of electrical signals.
2. Description of the Prior Art
Optical switches are known in the prior art that are fabricated from a body of semiconductor material including a pair of opposed major surfaces, on one of which is located a ground plane, and on the other of which is located a broken electrode microstrip transmission line, that is, a microstrip with a gap in the line. The gap may be completed by electrical charges generated in a surface region across the gap which is responsive to a beam of light. An example of such a switch is described in U.S. Pat. No. 3,917,943 of Auston. The semiconductor body in the Auston switch is characterized by uniform high resistivity which is sufficient to cause the semiconductor to behave as a dielectric with respect to electrical signals propagating along the microstrip transmission line. The device operates as a switch when a first optical beam is directed at the gap, thereby generating copious electronic charges at the surface of the semiconductor sufficient to increase significantly the conductance across the gap, and thus closing the electrical circuit at the gap between the broken portions of the electrode microstrip line. The optical beam is sufficiently intense to cause a sudden decrease of the d.c. resistance along the semiconductor surface across the gap to a value well below the characteristic impedance of the microstrip transmission line. As a result, the microstrip transmission line circuit is thereby closed (switched ON), enabling electrical signals to pass from an input electrical source to an output electrical detector located at opposed ends of the microstrip line. Then, a short predetermined time interval after commencement of the first optical beam, a second optical beam is directed at the gap in the microstrip line. This second beam causes the generation of copious electronic charges in the bulk of the semiconductor body down to the ground plane. The microstrip line at the gap is thus short-circuited to the ground plane (switched OFF), quickly terminating the flow of electrical signal energy from the input electrical generator to the output electrical detector.
The disadvantage of the Auston type optical switch known in the prior art is that the electrical impulse created by the optical pulse is one dominated by carrier recombination, rather than carrier transport.